The invention relates to residential branch circuit breakers, which are the molded case, narrow width type, usually 1 inch or 3/4 inch, such as shown in U.S. Pat. No. 3,081,386. These breakers are normally utilized in residential and commercial applications up to 240 volts.
Circuit breakers, and the panelboards or load centers to which they are mounted, are designed to be compact and physically compatible with existing apparatus. However, electrical utilities are providing increased available current in new installations. As a result, the short circuit interruption capacity requirement of narrow width circuit breakers has increased from 10,000 amps to 22,000 amps for new breaker designs, while existing apparatus designs place severe limitations on changes in physical arrangement and size.
In contrast, industrial breaker designs do not have such size constraints, and their much higher circuit interruption capacity involves different design criteria.
The present invention arose from efforts to increase the short circuit interruption capacity of a narrow width breaker. The interruption rating achieved exceeds the 22,000 amps requirement.
The invention involves the use of a single-piece nonlaminated slot motor mounted within the narrow width case and around at least one of the current carrying conductors. In preferred form, a unitary U-shaped slot motor is around both current carrying conductors, which conduct current in reverse parallel directions within the slot motor. The slot motor concentrates the magnetic flux density in the area of the parallel conductors to increase the blow open force between the conductors under short circuit conditions. The increased blow open force results in higher contact arm opening velocities and better interruption. The slot motor also helps to confine the arc and its debris to the area of the breaker case below the contacts. This is because the concentrated flux density near the conductor produces a high magnetic force which tends to push the arc and its debris off the contacts in a direction away from the breaker mechanism and towards the bottom of the narrow width breaker case. The pigtail of the breaker is outside of the slot motor, and any detrimental interaction between the slot motor current force and the other parallel current path blow open forces is not amplified.
In a desirable aspect, use of the slot motor is particularly cost effective. Relatively thin material, e.g. 0.063 inch thick relay steel, or other sheet metal, is used in the slot motor construction. The slot motor is coated with an arc-resistant material to minimize dielectric break-down problems.
The dramatically increased circuit interruption capacity is achieved within the narrow package, without resorting to wide case design or otherwise resorting to industrial breaker type design.